The long term objectives of this proposal are to understand the development and function of Johnston's organ (JO), which is the auditory organ in the genetic model organism, the fruit fly Drosophila. The Drosophila JO is homologous to the mammalian inner ear because they both rely for their specification on the activity of highly conserved transcription factors of the atonal family. The mouse atonal homolog 1 (Mathl) and the fly atonal genes can substitute for each other's function in reciprocal transgenic rescue experiments. Furthermore, the human atonal homolog 1(Atohl) can mediate regeneration of auditory hair cells in pharmacologically deafened mammals. Thus, the fly JO represents a powerful gene discovery resource for hearing. JO differs from other chordotonal organs in several ways that specialize it for hearing. At early pupal stages when many critical events of JO development occur, including asymmetric divisions of precursor cells, specification of sense organ cell lineages, and cell shape changes essential for correct differentiation, the JO is obscured within the puparium and very fragile to dissect. Our general strategy is to devise methods to better visualize the developing JO, and to use these methods as the basis for systematic expression microarray analysis for gene discovery. First we will characterize the roles of several JO genes at these early pupal stages. To characterize the medical relevance of these genes, we will screen for associations of their human homologs with families segregating deafness. Second, we will culture dissected antennal disks to image cell lineages and marker expression dynamically in vivo, away from the obscuring puparium. We will determine the fidelity of development in culture with several markers, and define the salient events at these stages. Third, we will exploit the higher throughput of this approach to recover sufficient RNA from wild-type and mutant antennal disks at these critical stages to compare gene expression using microarray analysis. We plan to use cut mutants initially as a paradigm. The cut transcription factor is required for normal JO development, and the mammalian homolog, CDP/Cux1 is expressed in the inner ear. Thus, we expect that the results of our experiments will permit us to identify target genes of cut that act at these critical stages. Overall, these studies will inform future research on the developmental and functional biology of the mammalian inner ear, and accelerate our understanding of human auditory disorders.